This invention relates to an apparatus for recording CRT image and, more particularly, to a CRT image apparatus which is designed so as to uniform exposure on a film when a CRT image is photographed.
Traditionally, an apparatus which allows optical images displayed on the CRT of a video monitor to be observed and the optical CRT images to be recorded b photographing on the film as desired is used in X-ray CT (Computer Tomography) systems for medical use. A camera having a mechanical shutter has been used to photograph a CRT image in such an apparatus. However, it is necessary in this case to place the camera in the vicinity of the CRT screen to photograph the CRT image. This causes a problem in that the raster scanning lines of the CRT image are photographed and, hence, the finished photograph is difficult to inspect for diagnostic purpose. To cope with this problem, techniques of increasing the number of scanning lines and obscuring the scanning lines by shifting them in the vertical direction (hereinafter referred to as "raster erase") have been used.
However, if an image displayed on the CRT of the video monitor, in which the raster erase has been performed, i.e., the scanning lines have been shifted, is photographed by a camera having a mechanical shutter, it may result in that the shutter closes before the scanning of one frame of the image is completed during exposure on the film, thereby causing local variations (nonuniformity) in the density of the photograph due to nonuniformity of exposure. To prevent this nonuniformity of exposure, increased exposure time may be required.
That is, if scanning has been performed up to an intermediate portion of the CRT screen, and if the number of times of scan in the area of the CRT screen, a difference in an optical density of the exposed and developed film (photograph) .DELTA.D is represented by the Equation: ##EQU1## where .gamma. represents the .gamma. value of the film used for photographing the CRT image, and is usually about 3.
Since the difference in the density which can be distinguished by eyes is at least 0.01, N=129 is obtained from Equation (1). The exposure time is usually about 2 seconds, so, if the CRT image is photographed for this time interval, N becomes less than 129, thereby causing local variations in density of the photograph due to nonuniformity of exposure. The time which corresponds to N=129 is about 17 seconds (If an 8-times raster erase is performed at a scanning rate of 528/60 Hz), resulting in an extension of the exposure time.
To prevent these local variations in density of the photograph due to the unevenness of exposure, one possible method involves opening and closing the shutter of the camera in synchronism with the frame of the CRT image. However, if the mechanical shutter is used for exposure on a film, as in the case of a conventional system, the CRT image is partially shaded by the shutter blades of the shutter as the shutter opens and closes. Therefore, it is difficult to completely eliminate the local variations due to the nonuniformity of exposure.